Vacuum-assisted countergravity casting methods, such as described in U.S. Pat. Nos. 3,900,064; 4,340,108 and 4,606,396, have been in use in the casting of a melt into a plurality of molds. Typically, the gas permeable molds are sealingly engaged in succession to a suitable vacuum housing and the underside of each mold so engaged to the vacuum housing is immersed in an underlying pool of the melt to countergravity cast the melt upwardly into the mold when a suitable differential pressure is established between the mold and the pool. A typical casting run may involve immersing approximately twenty (20) molds in the pool of the melt over a period of approximately five (5) minutes or more.
As the plurality of molds are cast from the common pool, the melt may become progressively contaminated with loose mold particulate and other foreign matter to an extent that may lead to production of unsatisfactory (dirty) castings and/or that may require replacement of the melt. Moreover, the casting process must be interrupted after a given number of molds (e.g., 20 molds) are cast to replenish the melt pool to the proper level for further casting. Periodic interruptions are thus required during the casting of a large number of molds (e.g., hundreds of molds) in a production run and adversely affect the economies of the production process. Still further, if the melt contains a fugitive alloyant, such as a volatile magnesium nodularizing agent in a grey iron melt, the concentration of the alloyant in the melt may decrease over the time period required to cast the plurality of molds from the common pool. For example, loss (or fade) of the volatile magnesium nodularizing agent from the iron melt can be as much as 0.075 w/o (i.e., weight percent) or more over the time required to cast the plurality of molds (e.g., twenty molds) from the common pool. Erratic, uncontrolled loss of the fugitive magnesium from the melt over time has been experienced and resulted in off-chemistry melts in so far as magnesium content is concerned and correspondingly inconsistent nodularization.
It is an object of the invention to provide an improved apparatus and method for the differential pressure, countergravity casting of a melt into a plurality of molds in such a manner that each mold is filled with an individual charge of the melt freshly admitted to a casting basin from a melt-holding chamber.
It is another object of the invention to provide an improved apparatus and method for the differential pressure, countergravity casting of a melt wherein each individual charge of the melt is admitted to the casting basin in an amount sufficient to cast only one of the molds such that the casting basin is supplied with a clean, fresh charge of the melt from the melt-holding chamber in order to cast the next mold.
It is another object of the invention to provide an improved apparatus and method for the differential pressure, countergravity casting of a melt into a plurality of molds wherein a fugitive alloyant (e.g., a magnesium nodularizing agent) is introduced into each individual charge in-situ in the casting basin just prior to casting each mold, thereby providing a freshly treated charge of melt for casting into each mold and substantially reducing loss (or fade) of the alloyant from the charge.
It is still another object of the invention to provide an improved apparatus and method for the differential pressure, countergravity casting of a melt wherein a large number of molds can be cast on a continual basis during a production run without major interruptions for melt replenishment.